Space programs such as the space shuttle program utilize reusable solid rocket boosters which are recovered from the ocean. Adequate protection must be provided for its steel and aluminum parts from heat damage during re-entry and from corrosion in the ocean environment. A low cost, thermal corrosion protective ablative coating, consisting of ten different ingredients, has been developed which includes phenolic microballoons, glass eccospheres, chopped and milled glass fibers to provide its primary insulative properties, an epoxy-polyurethane binder with a catalizer, a suspension agent, and an alcohol activator. Methylchloride and perchlorethylene are added to control viscosity.
The density of these ingredients lies between 0.64 and 1.08 gm/cc. Application of such a composition to create a homogeneous and smooth coating at a uniform thickness in an economical process is a problem which has been given considerable attention. Commercially available spray equipments cause the spray jet to disintegrate prior to impacting the workpiece. Heavy particles of the ablative material concentrate on the inside of the jet while light particles form clouds of fine mist on the outside of the jet causing an undesired overspray. The overspray has to be mechanically removed which upsets the required balance of material adversely affecting the material properties. The removal procedure frequently interrupts the spraying operation, resulting in increased labor costs. Overspray also causes debonding of subsequently sprayed layers which would jeopardize the hardware to be protected by the ablation material.
Attempts have been made to eliminate this overspray by confining the spray jet in metal tunnels and later in an air tunnel framed by four air film panels. It was thought this concept would prevent the spray ingredients from escaping the boundaries of a predetermined spray area but, in practice, a homogeneous spray jet was still not provided.
In attempts to remove the overspray from the workpiece, various mechanical concepts have been proposed. However, problems in positioning and cleaning the device made this method impractical.